Abstract [en]

We demonstrate sharp thermally induced switching between ferromagnetic and antiferromagnetic RKKY ( Ruderman-Kittel-Kasuya-Yosida) exchange in a spin-valve with the spacer incorporating a thin diluted ferromagnetic layer as the core. We illustrate the mechanism behind the effect as being due to a change in the effective thickness of the spacer induced by the Curie transition into its paramagnetic state. The ability to switch between ferromagnetic and antiferromagnetic states in a magnetic multilayer by a slight change in temperature may lead to new types of spin-thermoelectronic devices for use in such applications as memory or oscillators.

Holmgren, Erik

Abstract [en]

This thesis investigates the dynamics of spin vortex pairs in nanopillars andthermal effects in magnetic multilayers with RKKY-like indirect exchange.

Spin vortices are being investigated as storage elements for memory applicationsas well as for GHz oscillators and signal processing devices. In this work, spin vortex pairs in synthetic antiferromagnets (SAF’s) are studied. A SAF consists of two magnetic layers separated by a thin nonmagnetic spacer. The two magnetic layers can be set in to a vortex-pair state, one of which, with parallel core polarizationand antiparallel chirality (the P-AP pair-state), is rather unique as regards tomagnetostatics and spin dynamics. We show experimentally how the low temperaturecore-core hysteresis is modified by the presence of three types of asymmetryin the magnetic layers of the SAF: bias-field asymmetry, thickness imbalance, andcore pinning. Of special interest is that suitably designed core pinning can lift thedegeneracy of the different chirality states of the vortex pair.

We show that decoupling of strongly bound cores in a P-AP vortex pair can be resonantly enhanced by microwave fields of frequency matching the rotationalresonance of the pair at 3 GHz. As the excitation amplitude increases the pairexperience a period-doubling cascade, which eventually results in chaotic dynamics. Still higher microwave amplitudes result in dynamic deterministic decoupling, which takes place independently of thermal fluctuations. Introducing anharmonicity into the excitation reduces the duration required for the core-core decoupling by an order of magnitude. Defects within the magnetic layers modify the system’s spectral properties, which are in-depth investigated.

RKKY-like indirect exchange interaction in ferromagnetic/nonmagnetic multilayers is an interface effect, the sign of which depends on the thickness of the nonmagnetic spacer separating the ferromagnetic layers. This interaction is known to be insensitive to external control once the multilayer is fabricated. In this thesis, novel thermal control of indirect exchange coupling in specially designed multilayersis demonstrated. Two materials systems are investigated. The first incorporates a uniform Fe-Cr spacer separating two Fe layers, with the thickness chosen to correspond to the antiferromagnetic peak in the RKKY. Direct exchange across the spacer strongly couples the two Fe layers at low temperatures. Heating the system above the Curie temperature of the spacer results in antiferromagnetic indirect RKKY interlayer coupling. A rather strong magnetic proximity effect at the interfaces broadens the transition and weakens the indirect exchange. Introducing thin Cr layers at the two Fe-Cr interfaces suppresses the direct ferromagnetic exchange and sequentially transmits indirect RKKY exchange. We show that in this gradient-spacer multilayer design the thermal phase transition is significantly narrower, allowing thermal switching between indirect ferromagnetic and indirect antiferromagnetic exchange coupling of the outer magnetic layers.

We demonstrate, using RKKY exchange biasing, a reversible Curie transition in weakly ferromagnetic spacer layers. Thermal on/off switching of the spacer magnetization results in two different entropy states and a strong magnetocaloric effect. The low-field magnitude of the effect rivals that in many advanced bulk rare-earthbased materials, with the full effect being achieved in the 10 mT field range.